An expert in the mechanics of hand-to-hand combat who took the stand in the George Zimmerman trial on Wednesday testified that Mr. Zimmerman is a hapless fighter who could well have felt justified in shooting Trayvon Martin in self-defense.

Dennis Root, a private eye and a former cop with a background in fight dynamics, said that in a “dynamic combat event” there’s a “golden rule”: If you haven’t won the fight in 30 seconds, you need to change tactics.

The altercation between 17-year-old Trayvon and then-28-year-old Zimmerman had been under way for at least 40 seconds when Zimmerman grabbed his 9mm Kel-Tec and fired a straight shot into Trayvon’s chest. Forty seconds in a fight is “like a lifetime,” Mr. Root, a paid defense witness, told the six female jurors.

The question for the jury is whether Zimmerman was warranted in using deadly force, or whether he bears responsibility for engaging an innocent, unarmed teenager out of “ill will” and then fatally shooting him after he lost control of the situation.

Root acknowledged to the prosecution that Zimmerman had “other options” than to use his weapon, but doubted whether the defendant had the expertise to do much else to avoid further injury.

“We can sit here and come up with a thousand things that he may have been able to do,” he said. “But reality is what a person is perceiving to be their reality, and what his responses are for that given reality. There are always options in every force event, it’s just a matter of what you as an individual see as your options.”

Root acknowledged that Zimmerman had trained at a boxing gym, although he had never been deemed competent enough to face anybody in the ring. Zimmerman spent most of his time shadowboxing and grappling a sandbag. He “didn’t have the physical prowess to go into boxing another person. He didn’t have the skill level to physically interact with another boxer,” Root said.

That haplessness may have factored into Zimmerman's response, he added. "If I was dealing with Chuck Norris, I would expect a completely different response to any kind of physical altercation than I would if I was dealing with Pee-wee Herman," he said.

At one point, Root suggested that Zimmerman is more Pee-wee Herman than Chuck Norris. Earlier on Wednesday, Judge Debra Nelson barred cellphone messages from Trayvon suggesting that he was interested in mixed martial arts fighting.

“To the best of my knowledge, Mr. Martin was a physically active and capable person [while] Mr. Zimmerman is an individual who is by no stretch of the imagination an athlete,” Root said. “He would find himself lacking when compared to Mr. Martin.”

At the time of the encounter in February 2012, Trayvon was 5-foot-11 and weighed 158 pounds. Zimmerman was 5-feet-7 and weighed 185 pounds. Eyewitnesses have given contradictory accounts about which one appeared to be the aggressor in the fight. Also inconclusive is a recording of a 911 call in which a shriek of help can be heard. Trayvon's mother and Zimmerman's mother each testified under oath that she believed the voice to be that of her own son.

The fight dynamics testimony is important for the defense, which is trying to establish that Zimmerman had no choice but to shoot Trayvon, and that, as defense attorney Mark O’Mara said, “Trayvon Martin caused his own death” by needlessly attacking and beating George Zimmerman.

The defense was moving closer to conclusion in the high-profile trial of the former neighborhood watch captain, who the state alleges committed second-degree murder by exhibiting “ill will” toward Trayvon before shooting him to death.

Before leaving the stand, Root, a veteran cop, acknowledged that Zimmerman’s use of the word “subject” suggested that he saw Trayvon as a “bad guy.”

The state alleges that another snippet of a call Zimmerman placed to 911 – about “[expletive] punks” always getting away – suggested that the defendant harbored ill will toward Trayvon. It is necessary to prove ill will to win a second-degree murder conviction.

Earlier in his testimony, Root told Mr. O’Mara that the sequence of events that led Zimmerman to pull his gun and shoot Trayvon did not point to ill will, but rather to an urgent desire to stop an attack in which Zimmerman, according to physical evidence, was unable to land any blows on Trayvon.

Florida self-defense laws allow the use of deadly force to prevent death or grievous bodily injury. Prosecutors say Zimmerman waived those rights when he decided to pursue and confront Trayvon Martin against the advice of a police dispatcher.